In semiconductor devices such as a NAND flash memory device, memory cells are shrunk to achieve high integration. In case the memory cells are isolated by an element isolation region taking an STI (Shallow Trench Isolation) structure, the width of element isolation region becomes narrower with the shrinking of the memory cells. In contrast, the width of element isolation region between peripheral circuit transistors in the peripheral circuit region of a NAND flash memory device is significantly larger compared to the width of the element isolation region between the memory cells. Thus, in the element isolation region, the material of the element isolation film being filled into the element isolation trenches is preferably changed depending upon the dimensions such as the width or the depth of the element isolation trench, the shapes of nearby gate electrodes, and the voltage being applied to the elements, etc.
However, NAND flash memory devices have conventionally employed an approach in which the element isolation film was filled into the element isolation trenches between the memory cells and between the peripheral circuit transistors in the same process, meaning that element isolation insulating films made of the same material are filled into the respective trenches. As a result, local degradation in electrical properties was often observed. The degradation in electric properties originating from the material of the element isolation film may become even more prominent as the memory cells become even smaller.